Title: Primary vs. Secondary Crystallization Effects on Bond Strength in Thermoplastic Composites

Authors: Saeed Khaleghi, Mehran Tehrani

DOI:

Abstract: Thermoplastic composite bonding is central to joint integrity and structural performance. Fusion bonding relies on polymer chain mobility, interdiffusion, and entanglement across an interface above the melting temperature. Once these processes are complete, crystallization occurs during cooling and can enhance bond strength. This makes the relationship between thermal history, crystallinity, and bond strength especially relevant for modern manufacturing routes such as Automated Fiber Placement (AFP), welding, thermoforming, and other high-rate joining and consolidation operations that impose rapid, non-equilibrium thermal cycles. Under these conditions, crystallinity may develop primarily during cooling (primary crystallization) or be deferred to post-processing heat treatment (secondary crystallization), yet it remains unclear whether bond strength is governed more by the crystallization pathway or by the final crystallinity level at the interface when fusion is fully achieved. Bonded composite interfaces, utilizing carbon fiber reinforced low-melting polyetheretherketone (LM-PAEK), with distinct crystallization pathways were produced via slow cooling, rapid quenching, and rapid quenching followed by annealing. Thermal histories were verified in situ, and crystallinity evolution was quantified using differential scanning calorimetry. Lap-shear testing and SEM fracture analysis were then used to relate crystallinity to joint performance and fracture morphology. Results show that strength correlates more strongly with the attained crystallinity level than with whether crystals formed during cooling or during annealing, and that fast cooling followed by postannealing can achieve bond strength comparable to slow-cooled joints. The overall sensitivity of strength to crystallinity is modest, which is consistent with the shallow interdiffusion depth in fusion-bonded interfaces that limits the load-bearing interphase volume. SEM fractography shows a clear dependence of failure morphology on crystallinity, ranging from limited matrix deformation at low crystallinity to predominantly cohesive, plastically deformed fracture surfaces at higher crystallinity. Collectively, these findings support manufacturing-relevant strategies that pair rapid cooling with post-consolidation annealing to achieve robust fusion bonds in high-rate thermoplastic composite processing.

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Conference: SAMPE 2026

Publication Date: 2026/04/27

SKU: 207

Pages: 11

Price: $22.00